The aim of the study is to determine if quinidine sulfate will improve strength and reduce the severity of the neuromuscular transmission defects in patients with slow channel myasthenic syndrome. Changes in muscle strength and electrophysiological measures of neuromuscular transmission performed will serve as end points for the study. The protocol was initiated in 1996. Two of the patients enrolled to date discontinued quinidine sulfate after developing an allergic reaction characterized by fever, myalgias, and in one case transient elevation in liver transaminases. All symptoms resolved when the drug was discontinued. No other adverse effects were reported. Both of the patients who developed an allergic reaction to quinidine have been placed on Fluoxetine with a good clinical response and no adverse effects. The data from the first 6 patients with up to 18 months follow up has been presented at one national and one international meeting. Two manuscripts describing the work have been published (Annals New York Academy of Sciences, and Annals of Neurology). All patients enrolled to date have shown significant objective improvement in clinical and electrophysiological measures of strength and neuromuscular transmission function.